I would recommend substituting a granulated sweetener for the Natvia, as it provides some body to the brownie, and depending on the brand you use and the level of sweetness will determine how much to use. You could use Erythritol without adjusting the amounts as the sweetness level is the same. If liquid stevia is your preferred sweetener, I cannot be sure of the amounts, or how the brownie will turn out – it all comes down to experimentation. I hope this helps